The process of directional solidification involves the controlled cooling of castings so that they solidify continuously from one end of the mold to the other. Castings made by directional solidification either have columnar grain or a single grain as shown in U.S. Pat. No. 3,260,505 to Ver Snyder and U.S. Pat. No. 3,494,709 to Piearcey. The process has been applied with great utility to the manufacture of turbine blades for aircraft gas turbine engines. In this, the procedures for casting have been highly refined in order to optimize the yield and properties of castings. U.S. Pat. Nos. 4,190,094 to Giamei, 3,700,023 to Giamei et al, 3,931,847 to Terkelsen and patent application Ser. No. 409,785, now U.S. Pat. No. 4,450,889, to Grot generally indicate the procedures used.
In conventional casting, the metal solidifies in a matter of seconds or minutes inasmuch as the mold is substantially below the melting point of the metal. In contrast, in directional solidification the mold is in a furnace at a temperature above the melting point of the metal when the metal is introduced. Solidification is initiated at the end of the mold, usually due to the presence of a water cooled chill plate. The mold is then progressively withdrawn from the furnace to cause a thermal gradient to move along the length of the casting, thereby causing solidification. Casting times usually range from 10 minutes or more for a typical part.
There have been a number of problems connected with directional solidification which have been solved over the past 20 years. In large measure they have related to low yields of good castings because of grain structure or metallurgical interaction between the mold and metal.
The unique grain structure is the essence of directional solidified castings and the present invention is concerned with obtaining such. In columnar grain castings there are no grain boundaries transverse to the direction of the primary stress in a gas turbine blade. However, since a columnar grain part is still a polycrystal, the alloy of such parts contains grain boundary strengtheners to provide strength transverse to the grain direction. See U.S. Pat. No. 3,700,433 to Duhl. Single crystal castings have in essence one grain and permit the removal of the grain boundary strengtheners. This enables even higher strength as described below.
Like their polycrystalline forebearers, the alloys used in single crystal superalloy aircraft engine blades are nickel base and strengthened by a gamma prime precipitate. It is necessary to heat treat a single crystal superalloy casting at a high temperature after casting in order to obtain good properties. In the heat treatment at or above the gamma prime solvus, the casting is made more homogeneous to ameliorate the affects of the dendritic structure which is formed as a natural result of the casting procedure. Upon cooling a fine gamma prime precipitate is formed to provide the desired strength. As mentioned, it is the absence of the grain boundary strengtheners which enables single crystal superalloys to be heated to the required very high temperature without undergoing incipient melting. See U.S. Pat. No. 4,116,723 to Gell et al.
However, should there inadvertently be created a grain boundary in a single crystal casting, the casting is greatly weakened. Inasmuch as the common superalloy grain boundary strengtheners such as carbon, boron, hafnium, etc., are omitted, the grain boundary will exhibit extremely poor strength at elevated temperatures. The casting procedure is of course designed to avoid the creation of grain boundaries, in accord with the teachings of the patents referred to.
But, after casting the ceramic shell mold which adheres to the casting, and usually core material as well, must be removed from the casting. This is commonly undertaken by mechanical means and by caustic chemical immersion which attacks the ceramic but does not attack the metal. See U.S. Pat. No. 4,073,662 to Borom for a special example of such process. Next, the castings are subjected to the high temperature homogenization and solutioning heat treatment. If during the processing prior to this heat treatment the castings are hit against one another, or otherwise impacted, residual stress regions can be created. Upon exposure to the elevated temperature, these regions will be found to recrystallize. If this occurs in a region of the casting which is subjected to high stress (which comprises most aircraft blade regions) then the part must be discarded as unuseable because it may fail prematurely along the recrystallization formed grain boundaries. Single crystal superalloy castings are very expensive and such occurrences are to be avoided through substantial efforts. Despite efforts to minimize impact damage, recrystallization apparently attributable to this cause has been a continuing problem during the manufacture of experimental parts. While in experimental manufacture the resultant lower yields could be accepted (despite efforts to overcome the problem) now that single crystal superalloy turbine blades are coming into commercial production, there has been a greater need to solve the problem. The work which resulted in the invention was undertaken. But, recrystallization problems arose in the casting of parts that were normally processed and which could not be attributed to mechanical mishandling. The cause became mysterious. Recrystallization was noticed in the region where the airfoil portion of a blade transitions to a larger flange section. Recrystallization was also noticed in the vicinity of a slight gibbosity on the surface as a result of the use of soluble metal for holding delicate cores in place, in accord with the teachings of U.S. Pat. No. 3,596,703 to Bishop.
Basically, residual casting stress was not judged to be an apparent cause of the recrystallization. The inherent nature of the directional solidification process and the variability in occurrence of the recrystallization suggested this. There was a tendency to nonetheless attribute the recrystallization to untraceable impact damage during handling. The rationale was as follows: In conventional casting it is well understood that residual casting stresses arise principally in two ways: First, the metal solidifies relatively rapidly from the free surface inward, producing compressive stresses on the surface of the casting. Second, after solidification there is contraction of the metal which is greater than that of the typical ceramic mold, thereby producing thermal strains when there is mechanical restraint of the metal part within the ceramic mold.
But, in directional solidification these effects were not seen to be present in sufficient degree: First, the molten metal solidifies progressively along the length of the mold and there is no mechanism for producing compressive surface stresses. Second, the presence of substantial restraint was not apparent since the investment molds were relatively thin and fragile, and the metals very strong. Furthermore, the recrystallization associated with the pin residues and the variability in recrystallization from one production run to another introduced clouded any residual strain hypothesis. But certain recrystallized regions tended to extend substantially or totally through airfoil walls (typically 3 mm thick) suggesting a substantial rather than superficial effect.
Thus, prior to the present invention the cause of the recrystallization was a mysterious problem which several people in the technical community concerned with the production of superalloy gas turbine parts concerned themselves with.